Data Modeling Essentials 2005 phần 6 pdf

3. Some requirements may emerge only when the client has seen an actual design (“I like to sleep in complete darkness.” or “I don’t want to hear the kids practicing piano.”). The second extreme position is that we should develop a rigorous and complete statement of business requirements sufficient to enable us to develop and evaluate data models without needing to refer back to the client. For the reasons described above, such a comprehensive specification is unlikely to be practical, but there are good reasons for having at least some written statement of requirements. In particular: 1. There are requirements—typically high-level business directions and rules—that will influence the design of the conceptual data model, but that cannot be captured directly using data modeling constructs. We cannot directly capture in an E-R model requirements such as, “We need to be able to introduce new products without redesigning the system.” or, “The database will be accessed directly by end-users who would have difficulty coming to grips with unfamiliar terminology or sophisti- cated data structures.” 2. There are requirements we can represent directly in the model, but in doing so, we may compromise other goals of the model. For example, we can capture the requirement, “All transactions (e.g., loans, payments, purchases) must be able to be conducted in foreign currencies.” We can do so by introducing a generic Transaction entity class with appropriate currency-related attributes as a high level supertype. However, if there is no other reason for including this entity class, we may end up unnecessarily complicating the model. 3. Expressing requirements in a form other than a data model provides a degree of traceability. We can go back to the requirements documentation to see why a particular modeling decision was taken or why a particular alternative was chosen. 4. If only a data model is produced, the opportunity to experiment confi- dently with alternative designs may be lost; the initial data model effectively becomes the business requirement. Our own views have, over the years, moved toward a more formal and comprehensive specification of requirements. In earlier editions of this book we devoted only one section (“Inputs to the Modeling Task”) to the analysis of requirements prior to modeling. We now view requirements gathering as an important task in its own right, primarily because good design begins with an understanding of the big picture rather than with narrowly focused questions. In this chapter, we look at a variety of techniques for gaining a holistic understanding of the relevant business area and the role of the proposed 252 ■ Chapter 9 The Business Requirements Simsion-Witt_09 10/8/04 7:47 PM Page 252 information system. That understanding will take the form of (a) written structured deliverables and (b) knowledge that may never be formally recorded, but that will inform data modelers’ decisions. Data modeling is a creative process, and the knowledge of the business that modelers hold in their heads is an essential input to it. We do not expect to uncover every requirement. On the contrary, we soon reach a point where data modeling becomes the most efficient way of capturing detail. As a rough guide, once you are able to propose a “first cut” set of entity classes (but not necessarily relationships or attributes) and justify their selection, you are ready to start modeling. This chapter could have been titled “What Do You Do Before You Start Modeling?” Certainly that would capture the spirit of what the chapter is about, but we recognize that it is difficult to keep data modelers from modeling. Most of us will use data models as one tool for capturing requirements—and experimenting with some early solutions—during this phase. There is nothing wrong with this as long as modeling does not become the dominant technique, and the models are treated as inputs to the formal conceptual modeling phase rather than preempting it. Finally, this early phase in a project provides an excellent opportunity to build relationships not only with the business stakeholders but with the other systems developers. Process modelers in particular also need a holistic view of the business, and it makes sense to work closely with them at this time and to agree on a joint set of deliverables and activities. Virtually all of the requirements-gathering activities described in this chapter can prof- itably be undertaken jointly with the process modelers. If the process modelers envisage a radical redesign of business processes, it is important that the data modeling effort reflects the new way of working. The common understanding of business needs and the ability to work effectively together will pay off later in the project. 9.2 The Business Case An information system is usually developed in response to a problem, an opportunity, or a directive/mandate, the statement of which should be supported by a formal business case. The business case typically estimates the costs, benefits, and risks of alternative approaches and recommends a particular direction. It provides the logical starting point for the modeler seeking to gain an overall understanding of the context and requirements. In reviewing a business case, you should take particular note of the following matters: 1. The broad justification for the application, who will benefit from it, and (possibly) who will be disadvantaged. This background information is 9.2 The Business Case ■ 253 Simsion-Witt_09 10/8/04 7:47 PM Page 253 fundamental to understanding where business stakeholders are coming from in terms of their commitment to the system and likely willingness to contribute to the models. People who are going to be replaced by the system are unlikely to be enthusiastic about ensuring its success. 2. The business concepts, rules, and terminology, particularly if this is your first encounter with the business area. These will be valuable in estab- lishing rapport in the early meetings and workshops with stakeholders. 3. The critical success factors for the system and for the area of the business in general, and the data required to support them. 4. The intended scope of the system, to enable you to form at least a preliminary picture of what data will need to be covered by the model. 5. System size and time frames, as a guide to planning the data modeling effort and resources. 6. Performance-related information—in particular, throughputs and response times. At the broadest level, this will enable you to get a sense of the degree to which performance issues are likely to dominate the modeling effort. 7. Management information requirements that the system is expected to meet in addition to supporting operational processes. 8. The expected lifetime of the application and changes likely to occur over that period. This issue is often not well addressed, but there should at least be a statement of the payback period or the period over which costs and benefits have been calculated. Ultimately, this information will influence the level of change the model is expected to support. 9. Interfaces to other applications, both internal and external—in particular, any requirement to share or transfer data (including providing data for data warehouses and/or marts). Such requirements may constrain data formats to those that are compatible with the other applications. 9.3 Interviews and Workshops Interviews and workshops are essential techniques for requirements gathering. In drawing up interview and workshop invitation lists, we recommend that you follow the advice in Section 8.3 and include (a) the people whom you believe collectively understand the requirements of the system and (b) anyone likely to say, after the task is complete, “why wasn’t I asked?” Including the latter group will add to the cost and time of the project, and you may feel that the additional information gained does not justify the expense. We suggest you consider it an early investment in “change management”—the cost of having the database and the overall system accepted by those whom it will affect. People who have been consulted 254 ■ Chapter 9 The Business Requirements Simsion-Witt_09 10/8/04 7:47 PM Page 254 and (better still) who have contributed to the design of a system are more likely to be committed to its successful implementation. Be particularly wary of being directed to the “user representative”— the single person delegated to answer all of your questions about the business—while the real users get on with their work. One sometimes wonders why this all-knowing person is so freely available! 9.3.1 Should You Model in Interviews and Workshops? Be very, very careful about using data models as your means of communi- cation during these initial interviews or workshops. In fact, use anything but data models: UML Use Cases and Activity Diagrams, plain text, data flow diagrams, event diagrams, function hierarchies, and/or report layouts. Data models are not a comfortable language for most business people, who tend to think more in terms of activities. Too often we have seen well- intentioned business people trying to fulfill a facilitator’s or modeler’s request to “identify the things you need to keep information about,” and then having their suggestions, typically widely-used business terms, rejected because they were not proper entity classes. Such a situation creates at least four problems: 1. It is demotivating not only to the stakeholder who suggested the term but to others in the same workshop. 2. Whatever is offered in a workshop is presumably important to the stakeholder and probably to the business in general and will therefore need to be captured eventually, yet such an approach fails to capture any terms other than entity classes. 3. By drawing the model now, you are making it harder (both cognitively and politically) to experiment with other options later. 4. Future requirement gathering sessions focused on attributes, relationships, categories, and so on may also be jeopardized. Instead, you need to be able to accept all terms offered by stakeholders, be they entity classes, attributes, relationships, classification schemes, categories or even instances of any of these. Later in this chapter (Section 9.7), we look at a formal technique for doing this without committing to a model. Because “on the fly” modeling is so common (and we may have failed to convince you to avoid it), it is worth looking at the problems it can cause a bit more closely. In a workshop, the focus is usually on moving quickly and on capturing the “boxes and lines.” There is seldom the time or the patience to accu- rately define each entity class. In fact what generally happens is that each 9.3 Interviews and Workshops ■ 255 Simsion-Witt_09 10/8/04 7:47 PM Page 255 participant in the workshop assumes an implicit definition of each entity class. If a relationship is identified between two entity classes that have names but only ambiguous definitions (or none), any subsequent attempt to achieve an agreed detailed definition of either of those entity classes (which is in effect a redefinition of that entity class) may change the cardinality and optionality of that relationship. This is not simply a matter of rework: We have observed that the need to review the associated relationships is often overlooked when an entity is defined or redefined, risking inconsistency in the resulting model. You may recall that, in Section 3.5.8 (Figures 3.30 and 3.31), we pre- sented an example in which the cardinality and optionality of two relationships depended on whether the definition of one entity class (Customer) included all customers or only those belonging to a loyalty program. Similarly while a particular attribute might be correctly assigned to an entity class while it has a particular implicit definition, a change to (or refinement of) that definition might mean that that attribute is no longer appropriate as an attribute of that entity class. As an example, consider an entity class named Patient Condition in a health service model. If the assumption is made that this entity class has instances such as “Patient 123345’s influenza that was diagnosed on 1/4/2004,” it is reasonable to propose attributes like First Symptom Date or Presenting Date, but such attributes are quite inappropriate if instances of this entity class are simply conditions that such patients can suffer, such as “Influenza” and “Hangnail.” In this case, those attributes should instead be assigned to the relationship between Patient and Patient Condition (or the intersection entity class representing that relationship). 9.3.2 Interviews with Senior Managers CEOs and other senior managers may not be familiar with the details of process and data but are usually the best placed to paint a picture of future directions. Many a system has been rendered prematurely obsolete because information known to senior management was not communicated to the modeler and taken into account in designing the data model. Getting to these people can be an organizational and political problem but one that must be overcome. Keep time demands limited; if you are working for a consultancy, bring in a senior partner for the occasion; explain in concise terms the importance of the manager’s contribution to the success of the system. Approach the interview with top management forearmed. Ensure that you are familiar with their area of business and focus on future directions. What types of regulatory and competitive change does the business face? 256 ■ Chapter 9 The Business Requirements Simsion-Witt_09 10/8/04 7:47 PM Page 256 How does the business plan to respond to these challenges? What changes may be made to product range and organizational structure? Are there plans to radically reengineer processes? What new systems are likely to be required in the future? By all means ask if their information needs are being met, but do not make this the sole subject of the interview. Senior managers are far less driven by structured information than some data warehouse vendors would have us believe. We recall one consultant being summarily thrown out by the chief executive of a major organization when he commenced an interview with the question: “What information do you need to run your business?” (To be fair, this is an important question, but many senior managers have been asked it one too many times without seeing much value in return.) Above all, be aware of what the project as a whole will deliver for the interviewee. Self-interest is a great motivator! 9.3.3 Interviews with Subject Matter Experts Business experts, end users, and “subject matter experts” are the people we speak to in order to understand the data requirements in depth. Do not let them design the model—at least not yet! Instead, encourage them to talk about the processes and the data they use and to look critically at how well their needs are met. A goal and process based approach is often the best way of structuring the interview. “What is the purpose of what you do?” is not a bad opening question, leading to an examination of how the goals are achieved and what data is (ideally) required to support them. 9.3.4 Facilitated Workshops Facilitated workshops are a powerful way of bringing people together to identify and verify requirements. Properly run, they can be an excellent forum for brainstorming, for ensuring that a wide range of stakeholders have an opportunity to contribute, and for identifying and resolving conflicts. Here are a few basic guidelines: ■ Use an experienced facilitator if possible and spend time with them explaining what you want from the workshop. (The cost of bringing in a suitable person is usually small compared with the cost of the participants’ time.) ■ If your expertise is in data modeling, avoid facilitating the workshop yourself. Facilitating the workshop limits your ability to contribute and 9.3 Interviews and Workshops ■ 257 Simsion-Witt_09 10/8/04 7:47 PM Page 257 ask questions, and you run the risk of losing credibility if you are not an expert facilitator. ■ Give the facilitator time to prepare an approach and discuss it with you. The single most important factor in the success of a workshop is preparation. ■ Appoint a note-taker who understands the purpose of the workshop and someone to assist with logistics (finding stationery, chasing “no- shows,” and so forth). ■ Avoid “modeling as you go.” Few things destroy the credibility of a “neutral” facilitator more effectively than their constructing a model on the whiteboard that noone in the room could have produced, in a language noone is comfortable using. ■ Do not try to solve everything in the workshop, particularly if deep- seated differences surface or there is a question of “saving face.” Make sure the problem is recognized and noted; then, organize to tackle it outside the workshop. 9.4 Riding the Trucks A mistake often made by systems analysts (including data modelers) is to rely on interviews with managers and user representatives rather than direct contact with the users of the existing and proposed system. One of our colleagues used to call such direct involvement “riding the trucks,” refer- ring to an assignment in which he had done just that in order to understand an organization’s logistics problems. We would strongly encourage you to spend time with the hands-on users of the existing system as they go about their day-to-day work. Frequently such people will be located outside of the organization’s head office; even if the same functions are ostensibly performed at head office, you will invariably find it worthwhile to visit a few different locations. On such visits, there is usually value in conducting interviews and even workshops with the local management, but the key objective should be to improve your understanding of system requirements and issues by watching people at work and questioning them about their activities and practices. Things to look for, all of which can affect the design of the conceptual data model, include: ■ Variations in practices and interpretation of business rules at different locations ■ Variations in understanding of the meaning of data—particularly in interpretation and use of codes 258 ■ Chapter 9 The Business Requirements Simsion-Witt_09 10/8/04 7:47 PM Page 258 ■ Terminology used by the real users of the system ■ Availability and correct use of data (on several occasions we have heard, “Noone ever looks at this field, so we just make it up.”) ■ Misuse or undocumented use of data fields (“Everyone knows that an ‘F’ at the beginning of the comment field signifies a difficult customer.”) While you will obviously keep your eyes open for, and take note of, issues such as the above, the greatest value from “riding the trucks” comes from gaining a real sense of the purpose and operation of the system. It is not always easy to get access to these end-users. Travel, particularly to international locations, may be costly. Busy users—particularly those handling large volumes of transactions, such as customer service representatives or money market dealers—may not have time to answer questions. And managers may not want their own vision of the system to be com- promised by input from its more junior users. Such obstacles need to be weighed against the cost of fixing or working around a data model based on an incorrect understanding of requirements. Unfortunately, data modelers do not always win these arguments. If you cannot get the access you want through formal channels, you may be able to use your own network to talk informally to users, or settle for discussions with people who have had that access. 9.5 Existing Systems and Reverse Engineering Among the richest sources of raw material for the data modeler are existing file and database designs. Unfortunately, they are often disregarded by modelers determined to make a fresh start. Certainly, we should not incor- porate earlier designs uncritically; after all, the usual reason for developing a new database is that the existing one no longer meets our requirements. There are plenty of examples of data structures that were designed to cope with limitations of the technology being carried over into new databases because they were seen as reflecting some undocumented business requirement. But there are few things more frustrating to a user than a new application that lacks facilities provided by the old system. Existing database designs provide a set of entity classes, relationships, and attributes that we can use to ask the question, “How does our new model support this?” This question is particularly useful when applied to attributes and an excellent way of developing a first-cut attribute list for each entity class. A sound knowledge of the existing system also provides common ground for discussions with users, who will frequently express their needs in terms of enhancements to the existing system. 9.5 Existing Systems and Reverse Engineering ■ 259 Simsion-Witt_09 10/8/04 7:47 PM Page 259 The existing system may be manual or computerized. If you are very fortunate, the underlying data model will be properly documented. Otherwise, you should produce at least an E-R diagram, short definitions, and attribute lists by “reverse engineering,” a process analogous to an architect drawing the plan of an existing building. The job of reverse engineering combines the diagram-drawing techniques that we discussed in Chapter 3 with a degree of detective work to determine the meaning of entity classes, attributes, and relationships. Assistance from someone familiar with the database is invaluable. The person most able to help is more likely to be an analyst or programmer responsible for maintenance work on the application than a database administrator. You will need to adapt your approach to the quality of available documentation, but broadly the steps are as follows: 1. Represent existing files, segments, record types, tables, or equivalents as entity classes. Use subtypes to handle any redefinition (multiple record formats with substantially different meanings) within files. 2. Normalize. Recognize that here you are “improving” the system, and the resulting documentation will not show up any limitations due to lack of normalization. It will, however, provide a better view of data requirements as input to the new design. If your aim is purely to document the capabilities of the existing system, skip this step. 3. Identify relationships supported by “hard links.” Non-relational DBMSs usually provide specific facilities (“sets,” “pointers,” and so forth) to support relationships. Finding these is usually straightforward; determining the meaning of the relationship and, hence, assigning a name is sometimes less so. 4. Identify relationships supported by foreign keys. In a relational database, all relationships will be supported in this way, but even where other methods for supporting relationships are available, foreign keys are often used to supplement them. Finding these is often the greatest challenge for the reverse engineer, primarily because data item (column) naming and documentation may be inconsistent. For example, the primary key of Employee may be Employee Number, but the data item Authorized By in another file may in fact be an employee number and, thus, a foreign key to Employee. Common formats are sometimes a clue, but they cannot be totally relied upon. 5. List the attributes for each entity class and define each entity class and attribute. 6. The resulting model should be used in the light of outstanding requests of system enhancement and of known limitations. The proposal for the new system is usually a good source of such information. 260 ■ Chapter 9 The Business Requirements Simsion-Witt_09 10/8/04 7:47 PM Page 260 9.6 Process Models If you are using a process-driven approach to systems development, as outlined briefly in Section 1.9.1, you will have valuable input in the form of the data used by the processes, as well as a holistic view of requirements conveyed by the higher level documentation. The data required by individual processes may be documented explicitly (e.g., as data stores) or implicitly within the process description (e.g., “Amend product price on invoice.”). Even if you have adopted a data-driven approach, in which data modeling precedes process modeling, you should plan to verify the data model against the process model when it is available and allow time for enhancement of the data model. In any case, you should not go too far down the track in data modeling without some sort of process model, even if its detailed development is not scheduled until later. We find a one or two level data flow diagram or interaction diagram a valuable adjunct to communicating the impact of different data models on the system as a whole. In particular, the processes in a highly generic system will look quite different from those in a more traditional system and will require additional data inputs to support “table driven” logic. A process model shows the differences far better than a data model alone (Figures 9.1 and 9.2). 9.7 Object Class Hierarchies In this section, we introduce a technique for eliciting and documenting information that can provide quite detailed input to the conceptual data model, without committing us to a particular design. Its focus is on capturing business terms and their definition. The key feature of this technique is that no restrictions are placed on what types of terms are identified and defined. A term proposed by a stakeholder may ultimately be modeled as an entity class but may just as easily become an attribute, relationship, classification scheme, individual category within a scheme, or entity instance. This means that we need a “metaterm” to embrace all these types of terms, and since at least some in the object-oriented com- munity have stated that “everything is an object (class),” we use the term object class for that purpose. It is essential to organize the terms collected. We do this by classifying them using an Object Class Hierarchy that tends to bring together related terms and synonyms. While each enterprise’s set of terms will naturally differ, there are some high-level object classes that are applicable to virtually all enterprises and can therefore be reused by each project. Let us consider the various ways in which we might classify terms before we actually lay out a suggested set of high-level object classes. 9.7 Object Class Hierarchies ■ 261 Simsion-Witt_09 10/8/04 7:47 PM Page 261 [...]... our users to have the data model already in their minds, ready to be extracted with a few well-directed questions (“What things do you want to keep data about? What data do you want to keep about them? How are those things related?”) Unfortunately, much that is written and taught about data modeling makes this very naive assumption Experienced data modelers do not try to solicit a data model directly,... left blank Chapter 10 Conceptual Data Modeling “Our job is to give the client not what he wants, but what he never dreamed he wanted.” – Denys Lasdun, An Architect’s Approach to Architecture1 “If you want to make an apple pie from scratch, you must first create the universe.” – Carl Sagan 10.1 Designing Real Models Conceptual data modeling is the central activity in a data modeling project In this phase... design and evaluation stages The design of conceptual models is the most difficult stage in data model development to learn (and to teach) There is no mechanical transformation from requirements to candidate solutions Designing a conceptual data model 1 RIBA Journal, 72(4), 1 965 273 274 ■ Chapter 10 Conceptual Data Modeling Business Inputs Identify Requirements changes to requirements Design Solutions... class The extent to which this occurs will dictate how many 9.7 Object Class Hierarchies ■ 269 Class Source Synonym Administrative Area Country Definition Any area that may be gazetted or otherwise defined for a particular administrative purpose ISO 3 166 A country as defined by International Standard ISO 3 166 :1993(E/F) and subsequent editions Jurisdiction A formally recognized administrative or territorial... Part III, you will find data modeling structures for dealing with (for example) the time dimension, data warehousing, and the higher normal forms These structures are patterns that you can come to use and recognize Until relatively recently (as recently as the first edition of this book in 1994) there was little acknowledgment of the importance of patterns Most texts treated data modeling as something... first principles, and there were virtually no published libraries of data modeling patterns to which practitioners could refer What patterns there were tended to exist in the minds of experienced data modelers (sometimes without the data modelers being aware of it) That picture has since changed substantially A number of detailed data modelsgenerally aimed at particular industries such as banking,... Requirements Proposed Solutions changes to design Evaluate Solutions Figure 10.1 Selected Solution Data modeling as a design activity from first principles involves conceptualization, abstraction, and possibly creativity, skills that are hard to invoke on a day-to-day basis without considerable practice Teachers of data modeling frequently find that students who have understood the theory (sometimes in great... a single secret to getting over the problem of being stuck, it is that data modeling practitioners, like most designers, seldom work from first principles, but adapt solutions that have been used successfully in the past The development and use of a repertoire of standard solutions (“patterns”) is so much a part of practical data modeling that we have devoted a large part of this chapter to it We look... 2 76 ■ Chapter 10 Conceptual Data Modeling ■ ■ The deliberate production of alternatives, though this is by no means universal: many designers focus on one solution that seems “right” while recognizing that other solutions are possible The use of a central idea (“primary generator”) to help focus the thinking process: for example, an architect might focus on “seminar rooms off a central hub”; a data. .. “seminar rooms off a central hub”; a data modeler might focus on “parties involved in each transaction.” 10.3 Starting the Modeling Despite the availability of documentation tools, the early work in data modeling is usually done with whiteboard and marker pen Most experienced data modelers initially draw only entity classes and partly annotated relationships Crow’s feet are usually shown, but optionality . data stores) or implicitly within the process description (e.g., “Amend product price on invoice.”). Even if you have adopted a data- driven approach, in which data modeling precedes process modeling, . Object Class Hierarchies ■ 261 Simsion-Witt_09 10/8/04 7:47 PM Page 261 262 ■ Chapter 9 The Business Requirements Figure 9.1 Data flow diagrams used to supplement data models: “Traditional” model. Member Contribution Account Administration Fees Account Tax Account Member Contribution Administration Deduction Tax Deduction Employer Contribution be posted to be posted to be posted to be part. verify the data model against the process model when it is available and allow time for enhancement of the data model. In any case, you should not go too far down the track in data modeling without

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